1. Field of the Invention
The present invention generally relates to a pressure-contact type semiconductor device such as a Gate Commutated Turn-off (referred to as “GCT” hereinafter) thyristor, which is applied to, for example, a BTB, a SVG and the like for electric power applications, an inverter for driving an iron manufacturing roller and the like for industrial applications, and other high-voltage, large-capacity switches and the like.
2. Description of the Related Art
A conventional GCT thyristor will be described below with reference to FIG. 6. FIG. 6 is a cross-sectional view showing a main part of a GCT thyristor with a typical outer peripheral gate structure. Referring to the figure, an aluminum gate electrode 1a and an aluminum cathode electrode 1b are formed on an upper surface of a semiconductor substrate 1, and an aluminum anode electrode 1c is formed on a back surface thereof. Reference numeral 1d denotes an insulation film made of polyimide or the like, and reference numeral 1e denotes an insulating rubber member formed on the outermost periphery of the semiconductor substrate 1.
On the cathode electrode 1b on the top surface of the semiconductor substrate 1, a cathode distortion buffer plate 2 is provided, and an external cathode electrode (see FIGS. 1A and 1B) is formed on the outer top portion of the cathode distortion buffer plate 2. On the anode electrode 1c, an anode distortion buffer plate 4 is provided, and outside of it, an external anode electrode is formed under the anode distortion buffer plate 4. A ring gate portion 6 contacts the gate electrode 1a, and the contacting top end portion is a plane of about 0.5 mm width. An external gate terminal (see FIGS. 1A and 1B) electrically connects to the ring gate portion 6. The ring gate portion 6 is pressed to the gate electrode 1a by an annular elastic body having such a shape as a coned disc spring via an annular insulator, together with the external gate terminal. An insulator 10 insulates the ring gate portion 6 from the cathode distortion buffer plate 2 and the external cathode electrode. The GCT thyristor formed in this way has a sealed structure, the inside of which is substituted with an inert gas.
Next, an operation of the conventional GCT thyristor will be described. When the GCT thyristor is turned on, an electric current is flown from the external gate terminal to the external cathode electrode. A rising inclination of the gate current at this time is generally 1000 A/μs or more so as to speed up the turn-on expanding speed. When turned off, the current is flown from the external cathode electrode to the external gate terminal. At this time, it is required to supply a current with several thousands A/μs inclination for commutating the current equivalent to the main current to the gate. In order to supply such a large amount of current in an instant, a contact resistance of a current conducting path from the external gate terminal to the external cathode electrode is required to be reduced as much as possible.
Further, the GCT thyristor is generally used by applying a reverse bias voltage between the gate and the cathode. A gap between the cathode distortion buffer plate 2 and the gate electrode 1a formed on the surface of the semiconductor substrate has only several tens μm width. In order to prevent discharge in this gap, the insulation film 1d such as polyimide is formed on the innermost periphery and the surface of the gate electrode 1a to thereby coat the surface up to the position right below the insulator 10 (see, for example, Japanese Patent Unexamined Laid-open Publication No. 8-330572 (FIG. 1)).
However, in the conventional GCT thyristor as shown in FIG. 6, the ring gate portion 6 must contact the gate electrode 1a formed on the surface of the semiconductor substrate. Meanwhile, a portion of the gate electrode 1a positioned right below the outer periphery of the cathode distortion buffer plate 2, which is adjacent to the ring gate portion 6, must be coated with the polyimide insulation film 1d in order to prevent discharge.
Comparing to the aluminum electrodes and the polyimide insulation film formed by a photoengraving technique, the other fabrication members have larger dimensional tolerances. Further, in order to ensure a clearance for positioning each member at the time of fabricating, the positions of the cathode distortion buffer plate 2 and the ring gate portion 6 with respect to the semiconductor substrate 1 are varied within the range of the integration tolerance which is the sum of the dimensional tolerances of the respective members.
Moreover, the ring gate portion 6 and the outer peripheral portion of the cathode distortion buffer plate 2 adjacent thereto must be fabricated in an extremely narrow region of the outer peripheral portion of the semiconductor substrate. Therefore, it was extremely difficult to satisfy the condition that “a portion of the gate electrode 1a positioned right below the outer periphery of the cathode distortion buffer plate 2 must be coated with the polyimide insulation film 1d to prevent discharge in the gap”, considering from aspects of the component processing accuracy and the positioning accuracy when fabricating.
As another measure for preventing the discharge in the conventional structure, there is a method to remove the gap itself by decreasing the diameter of the cathode distortion buffer plate 2 and retracting it up to the outer diameter equivalent to that of the cathode electrode 1d formed on the surface of the semiconductor substrate. In this case, however, there arises a problem that dynamic characteristics (for example, a serge resistance) of the GCT thyristor degrades.